1. Field of the Invention
The present invention relates to a hard film with improved sliding characteristics and in particular having improved resistance to peeling (xe2x80x9cpeelingxe2x80x9d as used hereafter indicates that a portion of the surface of the film peels off.), and is effective on sliding surfaces of sliding members such as piston rings in internal combustion engines.
2. Description of the Related Art
Piston rings have been exposed to ever harsher operating conditions in recent years, due to higher engine output and compliance with exhaust emission regulations. Hard films utilizing physical vapor deposition such as a TiN film, a CrN film, a hard film containing oxygen in a solid solution state in a crystal structure of CrN (Japanese Patent Laid-open No. 6-265023), or a hard film containing carbon in a solid solution state in a crystal structure of CrN (Japanese Patent Laid-open No. 6-300130) have been proposed as countermeasures to cope with these harsh operating conditions. A hard film containing oxygen or carbon, or both oxygen and carbon in a solid solution state in a crystal structure of CrN and MO2N has further been proposed (Japanese Patent Laid-open No. 11-1763). A hard film of Crxe2x80x94Tixe2x80x94Nxe2x80x94O alloy (Japanese Patent Laid-open No. 6-346077) and a hard film of Tixe2x80x94Crxe2x80x94Nxe2x80x94O alloy (Japanese Patent Laid-open No. 6-346076) have also been proposed.
The above Japanese Patent Laid-open No. 6-346076 discloses a sliding member coated with a physical vapor deposition film comprised of titanium, a group of M (Cr, V, Zr, Nb, Mo, Hf, Ta, W and/or Al), nitrogen, and oxygen, in an atomic ratio of M/(Ti+M)=1 to 45%, O/(N+O)=5 to 80%. The above Japanese Patent Laid-open No. 6-346077 discloses a sliding member coated with a physical vapor deposition film comprised of chromium, a group of M (Ti, V, Zr, Nb, Mo, Hf, Ta, W and/or Al), nitrogen, and oxygen, in an atomic ratio of M/(Cr+M)=1 to 55%, O/(N+O)=5 to 80%.
Since the physical vapor deposition films of these inventions contain at least four types of elements, three independent composition formulae must be shown to define the composition range. However, the above two official gazettes show only two composition formulae. The ranges of absolute values for Ti, M or Cr are therefore not shown. Further, various configurations are possible for structures in the film comprised of titanium, a group of M (Cr, V, Zr, Nb, Mo, Hf, Ta, W and/or Al), nitrogen, and oxygen, in an atomic ratio of M/(Ti+M)=1 to 45%, O/(N+O)=5 to 80%, and also in the film comprised of chromium, a group of M (Ti, V, Zr, Nb, Mo, Hf, Ta, W and/or Al), nitrogen, and oxygen, in an atomic ratio of M/(Cr+M)=1 to 55%, O/(N+O)=5 to 80%. However, nothing is mentioned of these various configurations possible in the above two official gazettes.
The various types of technology have been proposed as described above. However, when the above physical vapor deposition films are subjected under harsh engine operating conditions to excessive repetitive strain due to sliding movement on their surfaces during use, cracks appear. These cracks develop in size and cause the problem of peeling to occur within the physical vapor deposition film.
In view of the above problems, the present invention has the object of providing a hard film with superior sliding characteristics and in particular having improved resistance to peeling.
The hard film of the present invention has a crystal structure comprised of mixed phases of CrN and TiN, and contains oxygen in a solid solution state in the CrN and TiN crystals. The hard film comprises chromium of 40 to 75 percent by weight, titanium of 10 to 40 percent by weight, oxygen of 0.5 to 15 percent by weight, and the remainder of nitrogen. The crystal particle size is less than 1 xcexcm.
The resistance to peeling deteriorates when the chromium content is below 40 percent by weight or exceeds 75 percent by weight. The resistance to peeling also deteriorates when the titanium is below 10 percent by weight or in excess of 40 percent by weight. The resistance to peeling also deteriorates when the oxygen content is below 0.5 percent by weight or in excess of 15 percent by weight.
The adhesion and the resistance to peeling in the film deteriorate when the crystal particle size is more than 1 xcexcm. A crystal particle size within 0.1 xcexcm is preferable.
Carbon may be used instead of oxygen in the above hard film, and may be contained in a solid solution state in a content of 0.5 to 10 percent by weight. The resistance to peeling deteriorates when the carbon content is lower than 0.5 percent by weight or in excess of 10 percent by weight.
Both oxygen and carbon may be contained in a solid solution state. In such a case, the oxygen content is 0.5 to 15 percent by weight, the carbon content is 0.5 to 10 percent by weight, and the total content of oxygen and carbon is within 20 percent by weight. The resistance to peeling deteriorates when the total oxygen and carbon content exceeds 20 percent by weight.
The hard film has preferably a Vickers hardness in a range of 1300 to 2300. The resistance to wear deteriorates when the Vickers hardness is less than 1300 and the adhesion deteriorates when the Vickers hardness exceeds 2300.
The sliding member of the present invention is covered on at least the sliding surfaces by the hard film. A piston ring for example covered with the above-mentioned hard film therefore has satisfactory sliding characteristics and in particular resistance to peeling under harsh operating conditions. The application of the hard film may be performed by ion plating.
In order to provide good adhesion, the sliding member has preferably an under film beneath the hard film. The under film is comprised of CrN not substantially containing oxygen and carbon in a solid solution state.
In order to provide good resistance to peeling, the crystals of CrN and TiN have preferably a preferred orientation of (200) surface or preferred orientation of (111) surface parallel to a surface being covered.
In order to provide good adhesion, the crystals of CrN and TiN have preferably a columnar structure developing in a columnar shape from the base material toward the film surface.
The thickness of the hard film is preferably in a range from 1 to 100 xcexcm.